Definitions

Lets talk terms for a moment.   Here are some terms associated with watches  from the period we're concentrating on -- say 1750 to 1900.

escapement - The escapement is the part of the watch movement  which controls the rate of transfer of power from the spring to the motion works  (ie. the gears that move the hands).  This consists primarily of the balance  staff, balance wheel, hairspring, and escape wheel, with other parts depending  upon the type of escapement you're dealing with.

verge - Also known as 'crown and verge', this is a type of  watch escapement, the precursor to the lever escapement which came into wide use  by the 1850s.   In a watch movement with a verge escapement, the flag-shaped  balance staff moves a 'verge' against a crown gear, releasing the power from the  spring to the motion works.  Verge movements are almost always fusee driven, but  fusee movements are not always verge escapements.

lever - The lever escapement, already mentioned above, was  invented by Thomas Mudge in the late 1750s and eventually supplanted the crown  and verge in English watches.   The lever escapement uses a rounded 'roller  jewel' (or a pin, in cheaper movements) set into a round plate on the balance  staff in parallel position to the balance wheel.   As the balance wheel rotates,  this jewel moves a lever which releases power from the spring to the motion  works.

detent - The detent escapement was developed in the latter part  of the 18th century and was used primarily in precision watches and marine  chronometers.  The detent escapement uses a combination of jewels which impulse  the balance in only one direction.  The first jewel is a roller jewel positioned  as in the lever escapement.   The second jewel, known as the detent, acts as a  momentary brake against the escape wheel.

duplex - The duplex escapement was developed in the mid-1700s  and was first used for more expensive watches.  The duplex escapement uses an  escape wheel which has two sets of teeth -- one for impulse and the other for  locking.    In later years duplex escapements came to be used primarily in less  expensive watches and 'dollar' watches.

cylinder - The cylinder escapement was developed around 1725  and was used more widely in continental movements.   Cylinder movements seen  today are generally associated with cheaper mass produced continental watches of  the later 19th century.   The name is derived from the construction of the  balance staff, which consists in part of a 'C'-shaped cylindrical portion which  engages rounded teeth on the escape wheel for power release.   Their construction  make these balance staffs very fragile, and most watchmaker's today will not  attempt to fashion a new one.

fusee - The fusee is a part of early watches which allowed even  transfer of power from the main spring to the escapement from the time it was  fully wound until the time it had wound down.   This innovation made it possible  for a watch to keep fairly accurate time all day, rather than running fast when  it was first wound and then running more slowly as the day progressed.   The  fusee itself is a roughly pyramid-shaped device around which (in watches) coils  a chain, the other end of which is connected to the spring barrel.   The fusee is  usually easy to see between the plates of the watch movement.  Fusee movements  were made into the late 19th century.

fusee chain - The chain used in fusee watch movements is  constructed rather like a length of minature bicycle chain with a hook at each  end.   These chains can be a bit fragile, and once broken they can be a challenge  to repair.   Broken chains can be repaired, however.   Fusee chains were made in a  wide variety of sizes, so if the chain for a fusee movement is missing it may  not be an easy task to replace it.   In general, new fusee chains are seldom  available from suppliers.   However, many collectors and repair-gurus keep a  supply of old ones on hand.   Maybe taken from that movement you just bought  without a chain...

balance cock - The balance cock is the 'arm' which holds the  balance wheel and staff in place in the movement.   These range from highly  decorated, diamond-jeweled circular fittings to fairly plain and unjeweled.    Balance cocks have been removed from watch movements and made into jewelry since  the Victorian era.   In general, they are not interchangeable and are nearly  impossible to replace.
 

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